Halogenated boron hydride anions are weakly coordinating anions and have been used as electrolytes and as catalytic components, particularly to enhance the catalytic activity of metal cations finding use in a variety of reactions. Typically, the halogenated boron hydrides have been the fluorinated, chlorinated, and brominated borohydrides.
The following references are illustrative of processes for the production of halogenated boron hydrides, including fluorinated borohydrides:
U.S. Pat. No. 3,551,120 discloses boron compounds of the formula Ma(B12H12-yXy)b where M is a cation having a valence of 1-4, and (B12H12-yXy) is a group which forms a divalent anion in an aqueous solution. The term M represents hydrogen, ammonium, and metal cations, e.g., groups I, II VIII, IIIb and so forth. X represents halogen, (F, Cl, Br, and I), carboxyl, nitro, nitroso, sulfonyl, and so forth. Example 1 shows the formation of Cs2B12H7F5 by effecting fluorination of CsB12H11OH in anhydrous HF.
U.S. Pat. No. 6,180,829 discloses metal compounds of polyhalogenated heteroborane anions of the formula M[RaZBbHcFdXe(ORxe2x80x3)f]k where M is a cation having a valence of from 1-4, e.g., an alkali or alkaline earth metal cation, R typically is a halogen or an alkyl group, Z is C, Si, Ge, Sn, Pb, N, P, As, Sb, and Bi; X is a halide and Rxe2x80x3 is a polymer, hydrogen, alkyl and the like. The subscripts represents integers. Example 2 shows the formation of the polyfluorinated monocarborane anion from a monocarborane hydride wherein CaCB11H12 is reacted with a mixture of HF and 10% F2 in N2. CsCB11F11H was recovered as a white solid. Significant cluster decomposition occurred during the fluorination, and yields were 50-60% at these loadings.
U.S. Pat. No. 6,448,447, a continuation-in-part of U.S. Pat. No. 6,180,829 and others, discloses in Example 11 the formation of K2B12F12 (1 g) by the continuous addition of a fluorine/nitrogen gas phase to a suspension of K2B12H12 in HF.
Knoth et al, Chemistry of Boranes, IX. Fluorination of B10H10xe2x88x922 and B12H12xe2x88x922 Inorganic Chemistry, Vol. 2, No. 2, Feb. 1964 disclose the preparation of highly fluorinated decaborates, by (a) effecting fluorination with anhydrous HF alone to a composition up to B12F6H62xe2x88x92, and, (b) effecting the direct fluorination of a 5 wt. % B12H122xe2x88x92 potassium salt by contacting the salt with F2 in the presence of water (under these conditions, the HF concentration is never  greater than 10% and thus the Hammett acidity, Ho remains  greater than 0 throughout the fluorination. (Gillespie and Liang)) The reaction when conducted in the presence of water was difficult to run to completion as evidenced by the use of a 5-fold excess of fluorine. In the end, a low yield (32%) of a hydroxy substituted fluoroborate, B12F11(OH)2xe2x88x92 was obtained rather than the desired fluorine substituted dodecaborate.
Sointsev, et al, Stereochemical Aspect of the Fluorination of the B12H12xe2x88x922 Anion, Russian Journal of Coordination of Chemistry, Vol. 23, No. 6, 1997, pp 369-376, disclose that the reaction of supercritical HF with K2B12H12 at 600xc2x0 C. generates the fully fluorinated anion. Significant decomposition was observed and yields of only 25% were obtained under these conditions.
This invention relates to an improvement in a liquid phase process for the direct fluorination of borohydride salts including the B10H10xe2x88x922 and B12H12xe2x88x922 salts to broadly produce compounds of the formula, Ma[RbZcBdHeFy]k where M is a cation having a valence of from 1-4, e.g., an alkali or alkaline earth metal cation, R typically is a halogen or an alkyl group, Z is C, Si, Ge Sn, Pb, N, P, As, Sb, or Bi; L; B is boron, H is hydrogen, a is 1 or 2; b is an integer from 0 to 7; c is an integer from 0 to 1; d is an integer from 5 to 12; e is an integer from 0 to 9; y is an integer from 3 to 12 and k is 1, 2 or 3, the respective values of a and k are determined by the valence of M, that is when the integer a is multiplied by the valence of M it is equal to the integer k or 2 times the integer wherein said borohydride salt is contacted with fluorine, under conditions for forming said fluorinated borohydride salt. The improvement in the liquid phase process resides in effecting the fluorination of said borohydride salt in the presence of a reaction medium having an acidity between that of water and neat HF. More specifically, the reaction medium should have a Hammett acidity, Ho, such that 0 greater than Ho greater than xe2x88x9211. In a preferred embodiment the reaction medium, e.g., the carrier should be capable of solubilizing the borohydride salt to provide loadings of 10% and greater by weight of the borohydride salt in the carrier. Preferred fluorinated compounds are MaB10H(10-y)Fy and (MaB12H(12-y)Fy, where y greater than 3 and more preferably y is at least 9 for the compound MaB10H(10-y)Fy and at least 10 for the compound MaB12H(12-y)Fy.
Several advantages can be achieved through the process described here. These include:
an ability to achieve high levels of fluorination of the borohydride anions, e.g., where y is 10-12;
an ability to achieve excellent yields and reaction efficiency;
an ability to achieve high loadings of reactant in the carrier; and,
an ability to minimize yield loss due to byproduct formation.